1. Field of the Invention
The present invention relates to a method for fabricating a large single-grained ferroelectric thin film, and more particularly, to methods for fabricating a large single-grained ferroelectric thin film grown by selectively nucleated lateral crystallization (SNLC) using an artificial nucleation seed, for fabricating a ferroelectric capacitor using the single-grained ferroelectric thin film, and for fabricating a ferroelectric memory device using the ferroelectric capacitor.
2. Description of the Related Art
In general, a Perovskite ferroelectric material including PZT (PbZrxTi1xe2x88x92xO3) possesses excellent piezoelectric, super-conductive and ferroelectric properties. Thus, PZT is widely used for various elements.
Recently, PZT applications are vividly under study as in a FRAM (Ferroelectric Random Access Memory) for storing information by using polarization of PZT deposited in the form of a thin film by means of sputtering, CVD (Chemical Vapor Deposition), sol-gel and so on, and a DRAM (Dynamic Random Access Memory) using a high dielectric constant.
In the case of a semiconductor memory device, as shown in FIGS. 1A and 1B, a basic structure of a single memory cell has an integrated form of a single transistor TR and a single capacitor C, in which the transistor and the capacitor in the memory cell are electrically connected with each other.
Also, the capacitor C has an upper electrode C1 and a lower electrode C2 on the upper portion and the lower portion of a dielectric D, respectively. The lower electrode C1 is connected with the transistor TR via a plug P. The gate and source of the transistor TR positioned on the lower side in a substrate S are connected to a word line and a bit line, respectively, to thereby play a role of controlling the operation of the capacitor C.
However, a conventional information memory device uses a ferroelectric thin film made of a polycrystalline PZT as a dielectric D of the capacitor C. Here, since such a polycrystalline structured ferroelectric thin film has a columnar structure, grain boundaries arranged perpendicular to the electrode function as a diffusion path of oxygen vacancy, resulting in a more accelerated degradation phenomenon, such as fatigue, retention, low breakdown field, etc.
Also, in the case of the conventional information memory device, applications of PZT have been limited owing to a fatigue phenomenon that lowers polarization whenever a process of storing and reading information is repeated, an aging phenomenon that lowers properties as time passes, and phenomena such as a low breakage electric field and a large leakage current.
Attempts to reduce fatigue have been currently made using an oxide electrode such as a RuO2 electrode instead of a Pt electrode as an upper/lower electrode in a ferroelectric thin film capacitor, in which case a leakage current relatively has increased.
Inventors have found that properties of a thin film can be enhanced since a degeneration expediting factor can be removed in the case that grain boundaries are excluded by selective nucleation and lateral growth in the result of studying properties of the conventional ferroelectric thin film.
Thus, the present invention provides a method applied to a capacitor in an actual semiconductor memory device in which the positions of the grain boundaries are adjusted to form a single-grained crystalline at a selected position in order to solve a property degeneration problem due to the grain boundaries.
To solve the above problems, it is an object of the present invention to provide a method for fabricating a single-grained ferroelectric thin film capable of growing a large single-grained ferroelectric thin film having an excellent thin film characteristic by selectively nucleated lateral crystallization (SNLC) using an artificial nucleation seed.
It is another object of the present invention to provide a method for fabricating a ferroelectric memory device using a ferroelectric thin film capacitor fabricated by forming an artificial nucleation seed at a position adjacent a plug for connecting a transistor and a capacitor, and growing a large single-grained ferroelectric thin film by selectively nucleated lateral crystallization (SNLC) using the artificial nucleation seed, in which excellent polarization is maintained even after undergoing a number of cycles, and a little amount of leakage current occurs, and a crystallization is possible at a low temperature to thereby maintain an excellent dielectric feature and lower an aging phenomenon such as a fatigue.
It is still another object of the present invention to provide a method for fabricating a ferroelectric thin film capacitor for use in a ferroelectric memory device in which the thickness of a growing single-grained ferroelectric thin film is grown in an ultra-thin film so as to be driven at a low operating voltage.
To accomplish the above object of the present invention, there is provided a method for fabricating a single-grained ferroelectric thin film comprising the steps of: forming a first conductive layer on one side of a semiconductor substrate, by using a conductive material; forming an artificial nucleation seed in an island form adjacent a position where a ferroelectric thin film is to be formed in the upper portion of the first conductive layer; forming a ferroelectric thin film on the whole surface of the substrate including the nucleation seed; and thermally annealing the ferroelectric thin film to thereby grow the ferroelectric thin film positioned in the lateral side of the nucleation seed into a single-grained ferroelectric thin film.
There is also provided a method for fabricating a ferroelectric memory device such as a ferroelectric random access memory (FRAM) using a ferroelectric thin film capacitor comprising the steps of: forming a transistor on one side of a semiconductor substrate; forming an insulation layer on the upper portion of the transistor; forming a plug for connecting the transistor and the capacitor via the insulation layer; forming a first electrode layer used as a first electrode of the capacitor on the upper portion of the insulation layer and the plug; forming an artificial nucleation seed at a position adjacent the plug on the upper portion of the first electrode; forming a ferroelectric thin film on the whole surface of the substrate including the nucleation seed; thermally annealing the ferroelectric thin film to thereby grow the ferroelectric thin film positioned in the lateral side of the nucleation seed into a single-grained ferroelectric thin film; and forming a second electrode layer used as a second electrode on the upper portion of the single-grained ferroelectric thin film.
Here, a p-type or n-type silicon substrate is used as the semiconductor substrate. The material used as the artificial nucleation seed is made of a ferroelectric thin film.
The ferroelectric thin film is not particularly limited but is made of a ferroelectric material of ABO3 perovskite composite, in which A is made of at least one selected from the group consisting of lead (Pb), barium (Ba), and strontium (Sr), and B is made of at least one selected from the group consisting of zirconium (Zr), titanium (Ti), lanthanum (La) and tungsten (W).
Also, the ferroelectric thin film is made of a ferroelectric material of Bi-layered super-lattices, such as, Bi4xe2x88x92xLaxTi3O12 (x=0xcx9c4), or Axe2x80x2BibMcO(2+3b+5c)/2 type composite including Bi, in which Axe2x80x2 is made of at least one selected from the group consisting of barium (Ba), strontium (Sr), and lead (Pb) and M is made of at least one selected from the group consisting of titanium (Ti), tantalum (Ta), and niobium (Nb).
The ferroelectric thin film can be deposited by one of sol-gel, sputtering and metal-organic CVD methods, and the crystallization of the ferroelectric thin film can be accomplished by a thermal annealing at 300xcx9c800xc2x0 C.
In this case, the crystallization thermal annealing can be accomplished by using one of a tubular furnace, a rapid thermal annealing (RTA) apparatus, and a scanning type rapid thermal annealing apparatus.
Also, the crystallization of the ferroelectric thin film proceeds laterally at the upper portion of the nucleation seed, and the size of the grain of the laterally grown single-grain thin film sets a thermal annealing time by a length necessary for the capacitor in the memory device.
Preferably, the electrode layer is made of at least one selected from the group consisting of platinum (Pt), iridium (Ir), iridium oxide (IrO2), ruthenium (Ru) and ruthenium oxide (RuO2), the plug is made of metal or a silicon-containing material and the insulation layer is made of a silicon-containing oxide or a polymer group material.
As described above, the present invention applies a new selectively nucleated lateral crystallization (SNLC) technique to fabrication of a ferroelectric memory device to prevent grain boundaries from being positioned in the fabricated memory device, in which the nucleation seed are artificially formed in the lateral side of the plug and then the ferroelectric thin film formed on the upper portion of the nucleation seed is laterally crystallized into a large single-grained ferroelectric thin film.
As a result, the present invention can fabricate a ferroelectric thin film capacitor and a ferroelectric memory device using the same, using a single-grained ferroelectric thin film, in which excellent polarization is maintained even after undergoing a number of cycles, and a little amount of leakage current occurs, and a crystallization is possible at a low temperature to thereby maintain an excellent dielectric feature and lower an aging phenomenon such as a fatigue.